The present invention relates to a spot size converter for coupling optical elements of different spot size with each other. The present invention relates also to a semiconductor laser module which combines the spot size converter and a semiconductor laser. The present invention relates also to an optical fiber laser device using the spot size converter.
Spot size converters have been used for efficiently transmitting light from one optical element to another, for example, from a semiconductor laser to an optical fiber, since these optical elements are different in their spot size with each other Referring now to FIGS. 9 to 11, a structure of a conventional spot size converter will he described In FIG. 9, the numeral 901 denotes a semiconductor laser. A laser light is radiated from a slender exit area 902 with a the width w and a thickness h (w greater than  greater than h) of the semiconductor laser 901. In FIG. 10, the numeral 1001 denotes an optical fiber which has a core extending through its center. An end face of the core which faces the semiconductor laser 901 forms a receiving area 1002 of a diameter xe2x80x9cdxe2x80x9d. Generally, the exit area 902 of the semiconductor laser 901 and the receiving area 1002 of the optical fiber 1001 fail to agree with each other. From this reasons a light coupling efficiency decreases in having directly coupled these optical elements. Therefore, the semiconductor laser 901 and the optical fiber 1001 are optically coupled together through the spot size converter 1101, as shown in FIG. 11. Spot size converters are generally comprised of a core and a cladding, which are different in refractive index with each other. In FIG. 11, however, the cladding is omitted for clarifying the illustration of the core. Therefore, in practice the numeral 1101 denotes the core of the spot size converter.
The spot size converter 1101 is aligned with the exit area 902 of a semiconductor laser in the receiving area 1102. On the other hand, the exit area 1108 is let be agreed with the receiving area 1002 of the optical fiber 1001. Therefore, the laser light introduced from the semiconductor laser 901 propagates toward the exit area 1103 while repeating total reflection in the spot size converter 1101. The spot size of laser light is converted into a size which meets the receiving area of the optical fiber 1001 as a result of the propagation.
The conventional spot size converter can convert the spot size of an optical beam. However, the conventional spot size converter cannot change a divergence pattern of light exactly. The divergence pattern of outgoing light from the spot size converter 1101 needs to be equal to or less than a light receiving pattern of the optical fiber 1001. As will be discussed later, the divergence pattern of the outgoing light from the spot size converter 1101 is determined by the divergence pattern of the incident light, i.e., the outgoing light from the semiconductor laser 901 and the shape of the spot size converter 1101
As shown in FIG. 9, let Sx-LD and Sy-LD be the width direction component and the thickness direction divergence component pattern 903 of the outgoing light from a semiconductor laser 901, respectively. As shown in FIG. 11, let w and h be the width and the thickness of the incident area 1102 of the spot size converter 1101. And, let dx and dy be the width and the thickness of the exit area 1103 of the spot size converter 1101. Then, let Sx-WG be the width direction divergence component 1104 of the outgoing light from the spot size converter 1101, while let Sy-WG be the thickness direction component thereof. When the side wall of the spot size converter 1101 is normal to the width direction axis (x-axis) and the thickness direction axis (y-axis) of the exit area of the semiconductor laser 901, the width direction axis (x-axis) and the thickness direction axis (y-axis) can be independently handled by approximation.
Generally, a slenderness ratio of the exit area of semiconductor lasers is very large. Therefore, the outgoing light from the spot size converter combined with such a semiconductor laser, width direction divergence component SX-WG becomes large rather than thickness direction divergence component Sy-WG Consequently, the width direction divergence component Sx-WG of the outgoing light lies off the acceptance pattern S-FB of the optical fiber combined with the spot size converter. Parts of the divergence pattern Sx-WG lying off the acceptance pattern S-FB fails to penetrate into the optical fiber. In semiconductor lasers, the higher the power is, the slanderness ratio of the exit area is larger. Therefore, the higher the power of the semiconductor laser is, a domain of the width direction divergence component Sx-WG of the light exiting from the spot size converter lying off the acceptance pattern S-FB of the optical fiber becomes larger. Therefore, the light coupling efficiency between a semiconductor laser and an optical fiber decreases.
As described above, the conventional spot size converter had the problem to which light coupling efficiency decreases, so that the slenderness ratio of the exit area of the optical elements by the side of the incidence was so large that the slenderness ratio of the exit area of a semiconductor laser was large.
An object of the present invention is to provide a spot size converter which is able to couple optical elements of different spot size with each other at a high-efficiency, a semiconductor laser module in a combination of the spot size converter and semiconductor laser, and an optical fiber laser device using the spot size converter.
According to the present invention the above object is achieved by a spot size converter according to claim 1. The deoendent claims are directed to different advantageous aspects of the present invention.
To achieve the above objects, a spot size converter according to a second aspect of the present invention is comprised of a lead-in section and a lead-out section, the inclined side wall is located on lead-out section, and letting w0 be the width or the incident area of the lead-in section, letting h0 be the thickness of the incident area, letting w1 be the width of the exit area of the lead-in section, letting h1 be the thickness of the exit area of the lead-in section, and letting Sw be a width direction divergence component of the light entering to the lead-in section, the width w1 and the thickness h1 of the exit area of the lead-in section are defined so as to satisfy the following equation.
sin (Sw)xc3x97(w0/w1)=sin (Sh)xc3x97(h0/h1)
Therefore, the lead-in section is able to make a nonaxisymmetric divergence pattern of the incident light axisymmetric. By introducing the light thus made axisymmetric into the lead-out section, the divergence pattern of the outgoing light from the spot size converter becomes smaller than that in default of such a lead-in section. Consequently, it is able to couple a light output from optical elements with a highly slender exit area to optical fibers at a high-efficiency.
Additional objects and advantages of the present invention will be apparent to persons skilled in the art from a study of the following description and the accompanying drawings, which are hereby incorporated in and constitute a part of the specification.